The present invention relates to a pliers type tool having an S-shaped return spring.
In several kinds of tools, such as pliers, return springs are used to bias the tool to an open state. It has, however, been shown that repeated use of the tool is less exhausting if the spring force does not increase linearly when the tool is closed, but is instead relatively constant or slowly diminishing, and for this purpose S-shaped springs are suitable. S-shaped springs are described in U.S. Pat. No. 4,801,019 and Swedish Patent 506 068. An adjustment of the spring force to the operator's wishes is then possible by locating the ends of the spring in various positions along the handles of the pliers.
Simple S-shaped springs with constant cross-section have some disadvantages, however, due to a strongly diminishing force when the deformation increases, and to an unstable position for a strongly compressed spring, allowing it to snap between two equilibrium positions where the curvature is stronger at one end or the other. With such springs, it may be necessary to use especially stiff connections to the handles of the pliers, such as keyhole shaped, as mentioned in U.S. Pat. No. 4,801,019.
The present invention concerns a spring with a more constant force and with a stable equilibrium position which does not deform the connections to the handles, and which makes it easier to handle small delicate items.